The special relationship of silicon to its thermal oxide SiO.sub.2 is responsible, to a large degree, for the successful manufacture and reliability of modern high density integrated circuits. This is especially true in the field of dynamic random access memories (dRAMs), where digital data is memorized in the form of stored charge across a capacitor. Thermal silicon dioxide serves as the conventional dielectric material for silicon to silicon capacitors, due to its low leakage current density, high electric field breakdown strength, high thermal stability, and low failure rate due to time dependent dielectric breakdown.
As integrated circuits, especially dRAMs, become more dense over time, the surface area available for the fabrication of an individual capacitor must necessarily decrease. In the field of dRAMs, however, the trend toward smaller capacitors runs counter to the desire to have the storage cell capacitance be as large as possible. Indeed, it is preferable that the storage cell capacitance for a dRAM have a value of at least 50 fF to keep the soft error rate due to alpha particle bombardment remain at an acceptably low level. Since the available surface area for the capacitor is decreasing with new generation technology, either the dielectric thickness or the dielectric constant must increase in order to maintain the storage cell capacitance at the desired value. With silicon dioxide as the capacitor dielectric, as the dielectric thickness is reduced, the extent of pinhole defects in the film increases, and the voltage of the onset of Fowler-Nordheim tunneling decreases. These factors point out the need for a dielectric material with a relative dielectric constant greater than that of silicon dioxide, so that the film thickness can remain at a manufacturable and reliable level, and so that the desired capacitance value can be attained in the reduced silicon surface area available.
Prior work has been directed to the use of materials other than silicon oxide, or in addition to silicon dioxide, as capacitor dielectrics in order to increase the relative dielectric constant for a capacitor. Examples of the use of sandwich films of silicon nitride with silicon dioxide are described in U.S. Pat. No. 4,577,390 issued Mar. 25, 1986 (oxide/nitride/oxidized nitride stacked film), and in copending application Ser. No. 174,751 filed Mar. 29, 1988 (nitride/oxide/nitride), both assigned to Texas Instruments Incorporated. Each of these examples provide for a dielectric film which has a higher effective dielectric constant than a film solely of silicon dioxide.
Materials other than silicon compounds have also been considered for use as a dielectric material. Yttrium oxide (Y.sub.2 O.sub.3) has been found to be a particularly attractive material, due to its high relative dielectric constant (13 to 16, as compared to 4 for silicon dioxide), as well as its relatively high electric field breakdown value (on the order of 4 MV/cm). The use of yttruim oxide as a dielectric material overlying silicon, and overlying a silicon dioxide film overlying silicon, is described by Gurvitch, Manchanda and Gibson in "Study of thermally oxidized yttrium films on silicon," Applied Physics Letters, 51(12) (September 1987), pp. 919-921, and by Manchanda and Gurvitch in "Yttrium Oxide/Silicon Dioxide: A New Dielectric Structure for VLSI/ULSI Circuits," IEEE Electron Device Letters, Vol. 9, No. 4 (April 1988), pp. 180-182. Where yttrium oxide is formed either over silicon, or over silicon dioxide over silicon, this work indicates that if the ambient temperature of the structure after formation of the yttrium oxide exceeds 500 degrees Celsius, silicon may react with the yttrium in the yttrium oxide.
The reaction of silicon with the yttrium in the dielectric has been found to be detrimental to the quality of the yttrium oxide dielectric film. Referring to FIGS. 1a and 1b, the electrical behavior of a capacitor having a dielectric of yttrium oxide formed directly over silicon is illustrated. The capacitor dielectric for which the results are shown in FIGS. 1a and 1b was formed by the sputtering of yttrium metal directly onto silicon, followed by rapid thermal oxidation to form the yttrium oxide, and followed by rapid thermal annealing at various temperatures. The upper plate of the capacitor is aluminum. FIG. 1a shows the plate to plate leakage current for positive bias polarity (upper plate to lower plate) for the various temperatures, and FIG. 1b shows the plate to plate leakage current for negative bias polarity for the anneal temperatures. It should be noted that the horizontal axis is defined as the "effective" electric field E.sub.eff in MV/cm so that the performance of the yttrium oxide can be compared to that of silicon dioxide. The effective electric field is defined as the applied electric field times the ratio of the dielectric constant of the particular material under test to that of silicon dioxide. The degradation in leakage with increasing anneal temperature is observed for positive polarity but not for negative polarity, indicating the presence of silicon in the capacitor dielectric, diffusing upwardly from the lower plate. The Gurvitch et al. paper referred to hereinabove clearly shows the presence of silicon in the yttrium oxide film when formed over a layer of silicon dioxide. This degradation with the temperature to which the dielectric film is exposed after its formation provides a serious limitation in the use of yttrium oxide in integrated circuits, as many processing steps at a temperature exceeding 400 degrees Celsius are necessary in the fabrication of a typical dRAM circuit after the formation of the storage capacitor. If yttrium oxide is used as the capacitor dielectric according to these prior configurations, either an increase in leakage due to such temperature exposure must be tolerated, or the capacitor must be formed at a late point in the manufacturing process. Neither alternative is desirable.
It is therefore an object of this invention to provide a method for forming an integrated circuit capacitor including yttrium oxide which provides improved thermal stability.
It is a further object of this invention to provide an integrated circuit capacitor using yttrium oxide in the dielectric which has improved manufacturing compatibility with modern processes.
Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to this specification in conjunction with the drawings.